•The mechanical properties of CuTa/CuTa nanolaminates are significantly dependent on the layer size.•Both the Hall-Petch and reverse Hall-Petch relationships are found with the change of layer ...thickness.•The interfaces play a significant role in obstructing the expansion of the plastic deformation region.•The interfaces dominate the deformation mechanism with the small-layer thickness.•The surface layer controls the nanoindentation behavior of the large-layer thickness.
The mechanical characteristics and deformation mechanisms of Cu80Ta20/Cu20Ta80 amorphous/amorphous nanolaminates under the nanoindentation are still unclear. In this study, the influences of layer thickness and interface on the indentation behavior of nanolaminates are systematically investigated using molecular dynamics simulation. Moreover, the effects of indenter size, loading velocity, and surface layer on the deformation mechanism of nanolaminates are also clarified. The important results expose a strong dependence on the layer size for the mechanical properties of nanolaminates. The hardness of nanolaminates is maximized for a specimen with a layer thickness of 17.5 Å. Besides, the hardness decreases with increasing the indenter radii due to the indentation size effect, and the hardness increases as the increase of loading velocity. The plastic deformation behavior displays that the interfaces play a significant role in obstructing the expansion of the plastic deformation region from the indentation into the interior of workpiece. Furthermore, the elastic recovery is highest for the sample with the smallest layer thickness of 5.0 Å, and the elastic recovery tends to increase with increasing the indenter radius and decreasing the loading velocity. The influence of surface layer shows that the interfaces dominate the deformation mechanism for specimens with small-layer thickness, while the surface layer controls the indentation behavior of nanolaminates with large-layer thickness.
The figure shows the physical model of CuTa/CuTa nanolaminates (a), variation of hardness versus layer thickness for the two nanolaminates (b), the shear strain dispersion for the two nanolaminates with different layer thicknesses (c and d). Display omitted
Abstract
Molecular dynamics is applied to explore the deformation mechanism and crystal structure development of the AlCoCrFeNi high-entropy alloys under nanoimprinting. The influences of crystal ...structure, alloy composition, grain size, and twin boundary distance on the mechanical properties are carefully analyzed. The imprinting load indicates that the highest loading force is in ascending order with polycrystalline, nano-twinned (NT) polycrystalline, and monocrystalline. The change in alloy composition suggests that the imprinting force increases as the Al content in the alloy increases. The reverse Hall–Petch relation found for the polycrystalline structure, while the Hall–Petch and reverse Hall–Petch relations are discovered in the NT-polycrystalline, which is due to the interactions between the dislocations and grain/twin boundaries (GBs/TBs). The deformation behavior shows that shear strain and local stress are concentrated not only around the punch but also on GBs and adjacent to GBs. The slide and twist of the GBs play a major in controlling the deformation mechanism of polycrystalline structure. The twin boundary migrations are detected during the nanoimprinting of the NT-polycrystalline. Furthermore, the elastic recovery of material is insensitive to changes in alloy composition and grain size, and the formability of the pattern is higher with a decrease in TB distance.
The mechanical and surface properties of non-equiatomic CoCrFeNiAl high-entropy alloy (HEA) in the scratching process are explored through molecular dynamics simulation in the current work. The ...result reveals that the loading force decreases with the reduction of the twin thickness because the presence of excessive twinning in the structure is obviously related to the material softening. The average coefficient of friction (COF) is not proportional to the increment of twin thickness, and the COF in the scratching of monocrystal HEA is greater than in the scratching of twinning-containing workpieces. The plastic deformation behavior shows that the development of shear strain and residual stress into the workpiece in-depth decreases with rising the twin boundary distance. In addition, the pile-up height is larger with the increment of twin thickness, showing the ease of material removal in the machining for the large twin boundary sample. The number of wear atoms of a monocrystal sample is significantly larger than that of the twinning-containing specimens. The microstructure evolution shows that the dislocation and the stacking fault grow along the {111} primary sliding system for the monocrystal specimen, while the defects develop along the twin boundary for twinning-containing specimens.
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•Mechanical and surface properties of non-equiatomic CoCrFeNiAl HEA are studied.•The loading force decreases with the reduction of twin boundary distance.•The friction coefficient in the scratching of monocrystal HEA is greater than that of twinning-containing samples.•Large twin boundary patterns are easier to remove material in the scratching.•The number of wear atoms of a monocrystal sample is larger than that of the twinning-containing specimens.
•Sliding of grain boundary and grain growth play a key role in the plastic deformation mechanism.•Atoms in a state of high stress-strain are located around the abrasive tip and in the grain ...boundary.•The reverse Hall-Petch relationship is observed within the grain size range of this study.•The ability to lose the material volume of a polycrystalline is larger than a single-crystalline.
The mechanical response of CuAlNi nanocrystalline under the nanoscratch through an abrasive tip sliding on the workpiece is investigated using molecular dynamics (MD) simulation. The influences of the grain size, alloy composition, temperature and scratch speed on the plastic deformation characteristic and wear mechanism are surveyed. The results represent that increasing the grain size leads to higher force and hardness, which suggests the reverse Hall-Petch relationship. Meanwhile, the indentation and scratch forces tend to increase when reducing the Cu content and temperature, increasing the scratch speed. The deformation behavior exhibits that grain boundaries play a key role in inhibiting the spread of strain and stress. The results show that the stress and strain are concentrated not only in the contact region between the abrasive tip and substrate but also in the grain boundary and adjacent grain boundary areas. Notably, the sliding, twisting of grain boundary and the fusion of grains are a significant mechanism in the deformation behavior of polycrystalline, resulting in the dislocation is strongly developed in the grain boundary. Furthermore, the movement of atoms in various directions leads to different morphology of pile-up. From quantitative results of the special wear rate show that the ability to lose material volume is larger with Cu86Al11Ni3 alloy and at a temperature of 600 K, as well as the polycrystalline is higher than the single-crystalline. Finally, the residual depth ratios exhibit more strain recovery at the grain size of 6.17 nm and lower temperature.
The figure shows the workpiece model (a) and the evolution of grain of CuAlNi alloy at different positions: (b1) before scratching, (b2) an indentation depth of 2.0 nm, (b3) a scratching distance of 1.0 nm, (b4) a scratching distance of 4.0 nm, (b5) a scratching distance of 8.0 nm. Display omitted
The figure shows the microstructure evolution of AlCoCrFeNi high-entropy alloy with different crystallographic orientations (a), twin boundary spacings (b), twin boundary inclination angles (c).
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•Microstructure evolution reveals the formation of Lomer-Cottrell and Hirth dislocation locks.•Both the Hall–Petch and inverse Hall–Petch relationships are observed with the change of twin boundary spacing.•Microstructure evolution and atomic flow are greatly dependent on the spacing and inclination angle of the twin boundary.•Surface morphology and wear volume depend on the microstructure of the material.
Surface nanotribological properties and subsurface damage of Al0.4CoCrFeNi high-entropy alloy during the nano-scratching processes are investigated using molecular dynamics. The results show that the surface wear characteristics and scratching-caused surface damage significantly depend on the crystallographic orientation, spacing and inclination angle of the twin boundary. For the variation of crystallographic orientation, the largest friction coefficient belongs to the crystallographic orientation 001, indicating that the movement of the indenter in this substrate is most restricted. The microstructure evolution reveals the formation of Lomer-Cottrell and Hirth dislocation locks because of the distinctness of angle between various slip systems. Both the Hall–Petch and inverse Hall–Petch relationships are observed for the difference of twin boundary spacing, and the maximum indentation force is achieved with a tilt angle of 0° resulting from the various interactions among the dislocations and twin boundaries. The microstructure evolution and the atomic flow are greatly dependent on the spacing and the inclination angle of twin boundary, where the twin boundary migration is the significant factor. Furthermore, the surface morphology is distinct between workpieces due to the elastic recovery at the surface, nucleation and slipping of dislocation, which implies that the wear volume depends on the material microstructure.
•Vibration-assisted scratching effectively decreases total force compared to conventional scratching.•Increase in amplitude and frequency leads to the reduction in total force, which is beneficial ...for scratching.•Increasing frequency causes an increase in scratched zone temperature, which has a positive effect on machining.•The scratching with a large vibration amplitude and frequency can remove more material.
Although vibration-assisted scratching achieves higher machining efficiency than conventional scratching, its microstructural behavior is still unclear. In the present work, the deformation behavior and microstructural growth of monocrystalline CoCrFeNiCu high-entropy alloy during the vibration-assisted scratching through molecular dynamics simulation are investigated. The expected result shows that the vibration-assisted scratching effectively decreases the total force compared to the conventional scratching. The increase in the vibration amplitude and frequency leads to the reduction in the total force, which is advantageous in scratching. The shear strain concentration region in vibration-assisted scratching is larger than that of conventional scratching due to the expansion of the scratched area. Besides, the increase in vibration amplitude and frequency leads to a corresponding increase in shear strain and residual stress. The vibration-assisted scratching produces a larger high-temperature zone than the conventional scratching, and the increase of vibration frequency also results in an increase in the scratched zone temperature, which has a positive effect on easier machining. The microstructure evolution shows that Shockley partial dislocations account for the majority of the total dislocations. Moreover, the scratching with a large vibration amplitude and frequency removes more material. Therefore, insights into scratching behavior at the atomic level can aid in the optimization of the vibration-assisted machining process.
The figure shows the simulation model (a), average force (b), surface morphologies (c). Display omitted
Molecular dynamics (MD) simulation is applied to investigate the mechanical response of AlCrCuFeNi high-entropy alloy (HEA) under the conventional cutting and ultrasonic elliptical vibration-assisted ...cutting (UEVAC). The influences of vibration frequency, amplitude ratio, and phase angle on the material removal mechanism are investigated. The results show that the strain and stress are concentrated on the contact area between the workpiece and cutting tool, as well as at the grain boundaries in both cutting methods. The temperature of workpiece under the UEVAC is significantly larger than the conventional cutting, which can have a positive impact on the phase transformation and makes UEVAC easier. The analysis of structure and dislocation exposes that the deformation behavior of polycrystalline is strongly influenced by the grain boundaries, the evolution of stacking fault and dislocation is obstructed by the grain boundaries. Moreover, the average cutting force of the UEVAC is reduced as rising the vibration frequency and amplitude ratio, while the average force under the UEVAC with various phase angles has no significant difference. The number of chip atoms shows that the material removal rate is greater under the UEVAC with a larger vibration frequency, lower amplitude ratio and phase angle. The plastic deformation of chip becomes more serious under the UEVAC with a vibration frequency of 150 GHz, amplitude ratio of 4, and phase angle of 75° due to the smallest cutting ratio.
The figure shows the physical model (a), von Mises stress clouds (b) and relationship of force-cutting length (c) of AlCrCuFeNi HEA under the conventional cutting (b1 and c1) and UEVAC with vibration frequency of 150 GHz (b2 and c2). Display omitted
•The mechanism of UEVAC and the conventional nano-cutting method are exhibited and compared.•Effects of vibration frequency, amplitude ratio and phase angle on deformation behavior of AlCrCuFeNi HEA are investigated.•The cutting forces of UEVAC and conventional nanocutting show significant differences.•The UEVAC can produce the higher material removal rate compared to conventional cutting.
The figure shows the physical property of Cu47.5Zr47.5Al5 alloy in the various structures: polycrystalline (a) and twinned polycrystalline (b).
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•GB and TB play a significant role in ...preventing the spread of deformation.•High stress is located in front of the indenter and in the vicinity of GB.•Plastic deformation is governed by the interaction of dislocation-GB-TB.•The inverse Hall-Petch relationship is observed with different grain sizes.
Molecular dynamics (MD) simulation is applied to study the plastic deformation response of the nanoscratching process via the investigation of an indenter sliding on the surface of CuZrAl nanocrystalline. The effects of different crystal structures, alloy compositions, grain sizes, and twin lamellar thicknesses are paid special attention. The results show that the force and hardness are decreased in the order of single crystalline, twinned polycrystalline and polycrystalline. The average force value during the scratching stage is larger when the Cu content decreases. Interestingly, the inverse Hall-Petch relationship is observed from the change of grain sizes and twin lamellar thicknesses. Furthermore, the grain boundary (GB), twin boundary (TB) and grain growth play a significant role in preventing the spread of deformation in the polycrystalline and twinned polycrystalline structures. The plastic deformation of single crystalline is controlled by the interaction of dislocations. Whereas the plastic deformation of polycrystalline is dominated by the interaction of dislocation and GB. For twinned polycrystalline, the interaction of dislocation-GB-TB has simultaneously occurred in the deformation process. A comparison of the special wear rates, this value is the largest with the polycrystalline structure in different crystal structures, while it tends to rise as decreasing the grain size.
Molecular dynamics simulations are employed to study indentation/scratching tests on the mechanical properties of Cu64Zr36/Cu amorphous/crystalline nanolaminates. The formation of narrow shear bands ...is the major cause of plastic deformation in the single indentation. In cyclic indentation, the deformation behavior has no significant difference with the various number of cycles. The cyclic load and hardness increase as rising the cyclic number. The hysteresis loop appears and is wider with increasing loading/reloading steps. In the scratching test, the shear deformation process occurs in the shear plane. The forces increase rapidly in the first stage; then the normal forces increase gradually, the tangential forces fluctuate horizontally. The friction coefficient increases higher with the increasing cutting depth; its value oscillates from 1.0 to 1.21.
The figure is shown the local stress distribution (a), the atomic configuration (b and c), and the temperature (d) of Cu64Zr36/Cu A/C nanolaminates under different processes. Display omitted
•The shear bands do not appear in the crystalline layers too thin.•The thicker crystalline layers effectively prevent the spread of stress and strain.•The friction between the indenter and substrate also induces a local heat gain.•The hysteresis loop appears and is wider as increasing unloading/reloading steps.•The shear deformation process occurs in the shear plane during the scratching test.
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•Nanoporous structure–mechanical property relations of 2D GaSe was reviewed by MD simulations.•The nanoporous 2D GaSe destructiveness drops by raising the tensiled neck ...width.•Mechanical parameters of the square-pore 2D GaSe decrease as increasing temperature.•Relative density is considered an essential parameter to determine material mechanical properties.
We apply molecular dynamics simulations to investigate the mechanical properties and atomistic deformation mechanisms of nanoporous gallium selenide (NPGS) nanosheets under uniaxial tension. The NPGS membranes structure–mechanical property correlation is investigated, especially the influences of temperature, neck width, pore shape, relative density, pore size, and strain rate. This work analyses the structural progression, initiation, fracture, brittle failure, Young’s modulus, ultimate strength, fracture strain, material toughness, and critical energy release rate. In most tensile cases, crack initiates at high-stress concentration regions such as pore edges or pore corners. The crack propagates in the perpendicular with tension direction and especially prefers to spread in the zigzag directions of membranes. We also find that the increasing temperature improves the atom kinetic energy, accelerating the fracture process and significantly reducing the material mechanical properties. The relative density is a particular essential parameter to determine material mechanical properties. Likewise, the size effect of neck width indicates both material characteristics, including “smaller is tougher” and “smaller is stronger”. Pore shape, notably diamond-pore, influences the stress distribution and stress absorption due to different tensile neck widths, leading to different mechanical responses, especially the material toughness. However, some material mechanical parameters were not much affected by some symmetrical pore shapes and strain rates. Remarkably, the stress distribution in the loading direction notably decreases the material mechanical performance. We also estimate the function between material mechanical properties and relative density or neck width as scaling laws to predict the mechanical properties of the NPGS nanosheets. The study results in an additional emphasis on mechanical behaviors and potentially expedites the promising applications of NPGS membranes.
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